The present invention generally relates to separation band detecting circuits in automatic program selection record player, and more particularly to a separation band detecting circuit used in an automatic program selection record player, capable of positively detecting separation bands on the record without being affected by external disturbance, noise, and the like.
Generally, a record contains plural pieces or programs of music, for example, on the same side of the record, and two consecutive pieces of music are separated from each other by a coarse groove portion. This coarse groove portion is the so-called separation band, and this separation band contains no recorded information. The separation bands are provided not only for providing a short interval between pieces of music, but also for facilitating the placement of a stylus of a phonograph or pickup cartridge on the record, so that a desired piece of music can be played back. However, in order to select and play the desired piece of music, the stylus of the pickup cartridge must be placed within the separation band, just prior to the desired piece of music. This selective placement of the stylus requires skill on the part of the operator who operates the record player. This is because the width of the separation band is relatively narrow, and the stylus may easily be erroneously placed on a wrong position. When the pickup arm is manipulated manually, for example, the stylus may be placed onto a fine-groove portion where the music is recorded, and the stylus and/or the record grooves may become damaged.
Recently, programmable automatic record players have been developed to eliminate the above described problems. In such a programmable automatic record player, the position of the pickup arm is detected by means using a photosensor. The pickup arm is automatically moved to the position of a desired piece of music which has been programmed, according to a command signal corresponding to the position of the pickup arm. Thus, the desired piece of music can be played back automatically. In this type of an automatic record player, the separation bands are detected and the number of these separation bands are counted, in order to detect the position of the pickup arm with respect to the recorded pieces of music. A device for performing such detection comprises a photodetector and a separation band detecting circuit.
As will be described hereinafter in conjunction with the accompanying drawings, the above photodetector comprises a light-emitting element and a photosensitive element. The light emitted from the light-emitting element is reflected at the surface of the record, and the reflected light is received at the photosensitive element. The manner in which the light is reflected differ in the fine-groove portions wherein a program signal of the music is recorded and the separation bands wherein no signal is recorded. Hence, the intensity of the light received by the photosensitive element differs according to whether the light is reflected at the fine-groove portion or the separation band. The change in the intensity of the received light is detected by the separation band detecting circuit, and the separation band is accordingly detected.
However, as will be described hereinafter in conjunction with the accompanying drawings, the conventional separation band detecting circuit has a disadvantage in that the separation band detecting circuit may be affected by external disturbance, noise, and the like, to perform erroneous detection. Generally, the record surface is not flat, and warps and undulations are introduced when the record is formed. Moreover, according to the magnitude of the recorded sound level, the width of the groove and the pitch of the groove vary. Furthermore, according to the state in which the record player is used, external light may reflect at the record surface. In this case, the reflected external light may reach the photosensitive element. Due to the various causes described above, the intensity of the light reaching the photosensitive element does not change between two values, between the recorded groove portion and the unrecorded groove portion in the separation band. That is, in reality, the light reaching the photosensitive element includes a noise component and a drift component which varies gradually. Thus, according to the setting of the threshold value of the separation band detecting circuit, the detecting operation may be affected by the above noise component and the drift component. In such a case, the separation band detecting circuit will erroneously detect a separation band although there is no such separation band.